The invention relates to surgical implants and, more particularly, to a method of manufacturing such implants from surgical grade austenitic stainless steel of the Fe-Cr-Ni type such as type 316L stainless steel.
Among the biocompatible alloys commonly used for surgical implants are titanium alloys, cobalt-chromium-molybdenum alloys, cobalt-chromium-tungsten-nickel, and nominally austenitic stainless steels of iron, chromium and nickel compositions. Of these materials, austenitic stainless steel is the most workable and least expensive starting material. The nominally austenitic Fe-Cr-Ni type is rendered corrosion resistant by surface passivation. Due to its work hardening ability and corrosion resistance, the Fe-Cr-Ni type stainless steel is particularly suitable for load bearing implants in the generally saline environment of the human body.
Many prosthetic devices such as hip prostheses must be formed to exacting size and shape specifications to fit the internal dimensions of the human bones. The austenitic stainless steels, because of their mechanical workability, are particularly advantageous for manufacturing these devices. In the past, prosthetic devices formed of austenitic stainless steel have been formed by heating the material to a high temperature such as 1750.degree. F. then hot forging to a final shape in a mold or machining it from a large block of material to a final shape and size. Heating austenitic stainless steel, however, results in a lower strength partly because the heat erases any cold-work that may be present.
Austenitic stainless steels are cold-worked to increase their mechanical strength. The cold-worked material is then used as a starting material for the manufacture of surgical implants. Additional strength improvement has been reported for one of the austenitic steels, namely Type 316L, by subjecting the cold-worked steel to a low temperature stress relief process, as discussed in "Improved Properties of Type 316L Stainless Steel Implants by Low-Temperature Stress Relief," by Hochman, et al, Journal of Materials at 425-442 (1966). The Hochman, et al article reports improvements in hardness, tensile strength, and yield strength by stress relieving cold-worked specimens of Type 316L stainless steel at temperatures of about 750.degree. F. (399.degree. C.) for approximately two hours. Although some improvement in mechanical strength of the cold-worked starting material has been achieved by this stress-relief technique, as reported by Hochman, the corrosion fatigue resistance of the stress-relieved starting material is not affected by such stress relieving.
It has also been reported that cold-working austenitic stainless steels reduces their corrosion resistance and therefore makes them more susceptible to pitting and corrosion fatigue in the generally saline environment of the human body. See, e.g. A. Cigada, et al, "Influence of Cold Plastic Deformation on Critical Pitting Potential of AISI 316L Steels in an Artificial Physiological Solution Simulating the Aggressiveness of the Human Body," J. Biomed. Mater. Res. 503 (1977); R. S. Brown, "The Three-Way Tradeoff in Stainless-Steel Selection," Journal of Mechanical Engineering, p. 59 (November, 1982); and B. Syrett, et al, "Pitting Resistance of New and Conventional Orthopedic Implant Materials--Effect of Metallurgical Corrosion," Vol. 34, No. 4, pp. 138-145 at p. 144 (April 1978). The conclusions appear to be based on corrosion tests of samples of the starting material which has been nominally cold-worked for the purpose of improving its tensile strength over that of the annealed starting material. However, as discussed below, data obtained regarding the life of an endoprosthesis manufactured in accordance with the present invention indicates improved performance even in a corrosive enviornment.
Casting the starting material has been considered in the past because it is less labor intensive and less expensive. But this option has been dismissed because cast material does not have suitable strength since the casting process results in a relatively porous material compared to a wrought material.
It is therefore desirable to provide a method for transforming a cast stainless steel implant into a finished device of suitable strength and corrosion resistance for use as a surgical implant. Such a method would provide implants with adequate properties that are cost effective for the elderly and less active patients.